1. Field of the Invention
This invention relates to an oxygen sensor for and a method of determining the concentration of oxygen (i.e., the partial pressure of oxygen) in a monitored gas environment.
2. Prior Art
One conventional oxygen sensor 10 shown in FIG. 1 comprises a diffusion housing 12, an oxygen ion conductive plate 14 of solid electrolyte hermetically bonded at one surface to the diffusion housing 12 by sealing glass 16, and a pair of circular electrode layers 18 and 18 secured to the opposite sides of the conductive plate 14, respectively, the electrodes 18 and 18 being connected to a DC power source 20. For example, the oxygen ion conductive plate 14 comprises a solid solution containing ZrO.sub.2, Y.sub.2 O.sub.3, MgO and CaO. Each of electrode layers 18 is porous and is made, for example, of platinum. The diffusion housing 12 has a gas diffusion aperture 12a of a small diameter formed through a wall 12b. The diffusion of oxygen from the monitored gas environment G through the gas diffusion aperture 12a into a chamber 12c of the diffusion housing 12 is effected by the application of a DC potential from the power source 20 across the two electrodes 18 and 18 to pump oxygen present in the chamber 12c through the oxygen ion conductive plate 14. As the potential across the two electrodes 18 is increased, electrical current flowing through the two electrodes 18 is changed as indicated by a curve A in FIG. 2. The curve A is divided into three regions X, Y and Z. The region X is a transition region at which the above-mentioned pumping of the oxygen out of the chamber 12c is in the process of reaching a constant rate. At the region Y, the amount of the molecules of the oxygen flowing into the chamber 12c through the gas diffusion aperture 12a is equal to the amount of the molecules of the oxygen flowing out of the chamber 12c through the oxygen ion conductive plate 14. At this region, the current limited by the oxygen diffusion is rendered stable so that a stable diffusion limited current value Ip is obtained. At the region Z, the current flowing through the two electrodes 18 is abruptly increased because the current contains components other than the ion current, such as current caused by the electronic conduction. The diffusion limited current value Ip is proportional to the concentration of oxygen in the monitored gas environment G, and therefore the oxygen concentration is detected by measuring the diffusion limited current value Ip through an ammeter A.
Another conventional oxygen sensor 10a shown in FIG. 3 differs from the oxygen sensor 10 of FIG. 1 in that a diffusion housing 12 made of an open-cell porous structure is used with the gas diffusion aperture 12a being omitted. With this construction, the oxygen diffuses into the housing chamber 12c through the porous housing 12, and the pumping of oxygen out of the chamber 12c is effected as described above for the oxygen sensor of FIG. 1.
The sealing glass 16 sealingly bonding the oxygen ion conductive plate 14 to the diffusion housing 12 comprises amorphous glass containing SiO.sub.2, BaO and Na.sub.2 O. The bond between the oxygen ion conductive plate 14 and the diffusion housing 12 must be maintained heat-resistant, airtight and thermal shock-resistant. The oxygen ion conductive plate 14 is made of zirconia, and the diffusion housing 12 is usually made of zirconia-based material so that there will not be provided a substantial difference in thermal expansion coefficient between the conductive plate 14 and the diffusion housing 12. Therefore, the diffusion housing 12 and the oxygen ion conductive plate 14 have a relatively high thermal expansion coefficient, for example, of 100.times.10.sup.-7 /.degree. C., and must have such a thermal resistance as to withstand up to 800.degree. C. For these reasons, the bond between the diffusion housing 12 and the oxygen ion conductive plate 14 must be made through glass. However, it is rather difficult to manufacture such glass as to meet these requirements. Glass containing SiO.sub.2, BaO and Na.sub.2 O is the only glass which is commercially available, partially amorphous and somewhat analoguous in physical properties to the above-mentioned desired glass. This amorphous glass has a poor thermal shock resistance, and a crack is liable to develop in it.